1. Field of the Invention
The present invention relates to a method for fabricating a transparent conductive ITO (Indium Tin Oxide) film, and, more particularly, to a method for fabricating transparent conductive ITO film which is utilized as an electrode of a liquid crystal display device, a solar battery, or the like by using a sputtering process.
2. Description of the Prior Art
A transparent conductive ITO film has been known which contains indium (In) and oxygen (O) as the basic component elements and tin (Sn) is added as a donor. Conventional methods for fabricating the transparent conductive ITO film include chemical film formation processes such as a spray process, a chemical vapor deposition (CVD) process, and a wet dip process, and physical film formation processes such as a vacuum evaporation process, and a sputtering process.
Among the above thin film fabricating methods, the sputtering process is superior to the others in that it can provide a transparent conductive ITO film with a relatively low resistance, and uniformly form a transparent conductive ITO film on a large substrate such as a glass plate.
There are direct current (DC) discharge-type sputtering processes and radio frequency (RF) discharge-type sputtering processes. The direct current discharge sputtering process (called the xe2x80x9cDC sputtering processxe2x80x9d) is conventionally used because it is inexpensive, provides stable discharge and has excellent controllability. A magnetron sputtering process is also conventionally used. This process converges plasma on the surface of a target with closed magnetic field generated by one or more magnets located behind the target. Because it has a high film formation rate, the magnetron sputtering process has excellent mass production capability. In view of the foregoing, a DC magnetron sputtering process which combines the direct current discharge process and the magnetron process is commonly currently used as the mass production apparatus for forming transparent conductive ITO films. Recently, a DC magnetron sputtering apparatus has been further developed to perform sputtering over the entire surface of the target by reciprocating (rocking) or eccentrically rotating the magnet located behind the target.
The sputtering process is known to have substrate temperature and oxygen partial pressure as factors affecting the resistivity of the resulting transparent conductive ITO film. As for the substrate temperature, a higher temperature provided a lower resistivity. For the oxygen partial pressure, on one hand the carrier density of the formed ITO film becomes higher in a region formed under a lower partial pressure because many oxygen vacancies exist in the film, while the mobility of the carriers becomes low. On the other hand, the carrier density of the ITO film formed in a region under higher partial pressure becomes lower as the number of oxygen vacancies decreases, while the mobility of carrier becomes higher. Because the resistivity is the inverse of the product of the carrier density and the mobility, there is optimal oxygen partial pressure where the resistivity is minimized in balancing between the carrier density and the mobility. Conventional sputtering processes have been used to fabricate a transparent conductive ITO film with low resistivity by adjusting the substrate temperature and the oxygen partial pressure as parameters. A sputtering process is disclosed in Japanese patent publication HEI 2-47255, herein incorporated by reference, wherein an oxide thin film is manufactured in a period in which only an inert gas (argon) is introduced and in other periods only oxygen gas is introduced.
In the conventional sputtering process, there is a problem that the resistivity of the transparent conductive ITO film formed on each substrate gradually increases as the number of the substrates on which the ITO film is formed increases. In a continuous sputtering process, a transparent conductive ITO film having the same resistivity cannot be formed on each sequentially processed substrate. That is, the continuous sputtering process causes a problem in that the resistivity increases as sputtering proceeds to sequentially form the ITO film on substrates.
FIG. 2 shows a graph of the characteristics of the change in the films resistivity to cumulative power supplied to the target when transparent conductive ITO films are formed on a plurality of substrates by the continuous DC magnetron sputtering process. In the example, the target was a disk-shaped mixed sintered material (density of 95%) of In2O3 to which 10 wt % of SnO2 is added and having a diameter of 8 inches (xcfx86 8xe2x80x3). The ITO film was formed on a substrate heated to 200xc2x0 C. at a power of 300 W and under a pressure of 3xc3x9710xe2x88x923 Torr. The sputtering gas was a mixture of argon (Ar) and oxygen (O2) gases. The partial pressure of the oxygen in the sputtering gas was adjusted to be within a range of from 6xc3x9710xe2x88x925 to 1.5xc3x9710xe2x88x924 to minimize resistivity every time a constant power (3 kWh) is cumulated. As clearly seen from FIG. 2, the resistivity of the film increased as the cumulative power increased.
When transparent conductive ITO films are formed on a number of substrates by the continuous DC magnetron sputtering process, the conventional sputtering process cannot provide uniform resistivity for each substrate, as shown in FIG. 2. In conventional film formation processes, to maintain the resistivity of each substrate to within a range where the performance of the device is guaranteed, it is necessary to grind the surface of the target, or to replace the target itself before the potential end of its life. This prevents the productivity from being improved, so that the manufacturing cost is increased.
Although FIG. 2 shows that the resistivity increases as the cumulative power increases in a case where transparent conductive ITO films are formed on a plurality of substrates by continuous sputtering, strictly speaking, such increase of the resistivity of the film for the cumulative power may occur even when forming a film on one substrate. That is, the resistivity gradually increases in the direction of film thickness even on a transparent conductive ITO film formed on one substrate. Therefore, when it is intended to form a film with considerable thickness on a substrate, there also arises a problem that the resistivity of entire film increases as the film becomes thicker.
An object of the present invention is to provide a method capable of fabricating a transparent conductive ITO film with resistivity within a predetermined range until the end of the life of the target when transparent conductive ITO films are continuously formed on a plurality of substrates by continuous sputtering, and capable of fabricating a transparent conductive ITO film with resistivity being within a predetermined range even when the film becomes thicker in forming it on a single substrate.
To attain the above object, the method for fabricating a transparent conductive ITO film according to the first feature of the present invention is a method for fabricating a transparent conductive ITO film which has In and O as basic component elements and which is added with Sn as a donor, in an atmosphere of a mixture of rare gas and oxygen gas by a sputtering process using a mixture of oxides of In and Sn as a target wherein it comprises a first step of forming a transparent conductive ITO film on a substrate, and a second step of interrupting the first step and performing discharge in a state with a partial pressure of oxygen higher than that in the first step to compensate for the oxygen deficiency in the target, the first and second steps being repeated alternately.
The method according to the second feature of the present invention is a method according to the first feature wherein a transparent conductive ITO film is continuously formed on each of a plurality of substrates by the first step, and then the second step is performed.
The method according to the third feature of the present invention is a method according to the first feature wherein the first and second steps are repeated alternately for one substrate.
The method according to the fourth feature of the present invention is a method according to any one of the first to third features wherein the partial pressure of oxygen in the second step is 1xc3x971031 3 Torr or more.
The method according to the fifth feature of the present invention is a method according to the fourth feature wherein the second step is performed in an atmosphere where introduction of the rare gas is stopped and only oxygen gas is introduced.
The method according to the sixth feature of the present invention is a method according to any one of the first to fifth features wherein the process interrupts the first step when the resistivity of the transparent conductive ITO film becomes higher than a set value, and enters in the second step, performs the second step for a predetermined period of time, then restarts the first step, and alternately repeats the first and second steps in the same manner.
According to the first feature of the present invention, because oxygen deficiency in the target is compensated in the second step where oxygen is deficient due to film formation in the first step, the amount of active oxygen generating from the target is restored when the transparent conductive ITO film is formed again by the first step, so that the resistivity of the formed film is lowered.
According to the second feature of the present invention, because the second step is arranged to be performed after the film formation by the first step in continuously forming on a plurality of substrates by continuous sputtering, it possible to lower the resistivity of the film formed on each substrate so that it is maintained to be within a desired range.
According to the third feature of the present invention, because the first and second steps are repeated alternately for a single substrate, it is possible, in forming the film on a single substrate to lower the resistivity by reducing the thickness of the film.
According to the fourth feature of the present invention, because the partial pressure of oxygen in the second step is set to 1xc3x971031 3 Torr or more, the following effects occur.
For example, the etch rate at a portion of a target which is most etched in fabricating a transparent conductive ITO film by the DC magnetron sputtering process is as much as 20 nm/sec depending on the power applied to the target, or the intensity or shape of the magnetic field over the target surface. This speed is about 100 atomic layers per second since the inter-atomic distance of ITO is about 2 xc3x85. The probability of sticking oxygen to the target can be estimated to be about 0.1 from the data on oxygen stacking to the oxide surface and the process of experiments in developing the present invention. Then, about 100 atomic layers would be formed in one second at the oxygen partial pressure of 1xc3x9710xe2x88x923 Torr (the oxygen does not actually form a layer, but it is assumed that one is formed), which is substantially equal to the etching rate for the target. Therefore, if the oxygen partial pressure is increased to over 1xc3x9710xe2x88x923 Torr, the stacking rate of oxygen on the target surface becomes higher than the rate of the release of oxygen by the selective sputtering of oxygen atoms or rise in surface temperature.
In addition, the target surface has higher temperature during sputtering due to ion bombardment. Thus, while some of the oxygen particles sticking on the target surface or oxygen particles ionized in the plasma, bombarding the target and remaining therein would be released, the remainder would diffuse into the target. As described above, by sputtering at the oxygen partial pressure of over 1xc3x9710xe2x88x923 Torr, oxygen can be sufficiently supplied to the oxygen-deficient layer superficially formed in the target during film formation, so that the oxygen concentration in the deficiency layer can be restored to substantially equal to the initial value. Thus, the resistivity which was increased during film formation is returned to the initial value.
According to the fifth feature of the present invention, the oxidizing speed of the target can be increased by stopping the introduction of rare gas causing selective sputtering, which is a cause of oxygen deficiency on the target surface, during the second step for compensating for oxygen deficiency in the target, and by performing discharge only with oxygen gas so that the time for the second step for compensating oxygen for the oxygen-deficient layer can be shortened.
According to the sixth feature of the present invention, the process interrupts the first step, based on the fact that the resistivity of the transparent conductive ITO film fabricated in the first step becomes higher than the set value, proceeds to the second step which compensates for the oxygen deficiency in the target, and repeats the first and second steps so that the transparent conductive ITO films having a resistivity of within a predetermined range can be obtained until the end of the life of the target even if they are fabricated on a plurality of substrates by continuous sputtering. Also, in the case of film formation on a single substrate, it is possible to obtain the transparent conductive ITO film having a resistivity of within a predetermined range even if the film thickness is increased.